This invention relates to an exposure time control device for an electric shutter.
In a single reflex camera of the "through the lens" photometric type, it is impossible to determine the brightness while the shutter is in operation. Thereupon, it has been proposed that all the while the shutter is in operation the brightness value of a photoconductive element at the time just before said shutter is operated is memorized and the exposure time is controlled automatically on the strength of said memorized value.
In such a proposition, after applying a logarithmic compression by producing the output voltage proportional to the logarithmic value of the illuminance on the light receiving surface of a photoconductive element, by means of a circuit in which photographic conditions such as setting diaphragm value, film sensitivity, etc. are set up as a bias voltage, said output voltage applied with said logarithmic compression and the bias voltage are operated photographically to allow a condenser to memorize the proper exposure time, and the memorized voltage in said condenser is applied with an inverse logarithmic conversion again to effect the automatic exposure control. And accordingly, it has the advantage of that it is possible to allow a memory condenser to memorize covering all sphere of the brightness of an object in a wide range, however, said memorized voltage is applied with a logarithmic compression so that if there should be a small error therein, in the process for applying an inverse logarithmic conversion said small error is enlarged resulting in a broad error in the exposure time.
Especially in such a proposition, behind the circuit for applying the logarithmic compression there is provided the circuit for setting up photographic conditions such as diaphragm value, film sensitivity, etc. as the bias voltage, so that in respective circuit it is necessary to do the voltage compensation and temperature compensation, and yet if this is imperfect an error which is not negligible may come out in the exposure time.
Anyway, in this manner giving the voltage compensation and the temperature compensation to respective circuit results in a very complex circuit as a whole and yet the perfect compensation can not be expected.
On the other hand, in such a circuit the exposure control is effected by an electromagnet, and while the coil of said electromagnet is in operation a large current runs therethrough so that the voltage variation of power source before and after the shutter operates is remarkable. Therefore, at the photometric time when the electromagnet coil is not operated, even though the illuminance on the light receiving surface of a photoconductive element is memorized by compressing so as to be in proportion to its logarithmic value, the power source voltage undergoes a change at the shutter operating time when the electromagnet coil is operated, therefore, when the memorized voltage memorized by compressing is applied with the inverse logarithmic conversion, on account of said voltage variation of power source an exposure control error which is not negligible comes out, and this fact stands in the way to put such a propostion to practical use.